This invention relates to a timepiece, and more particularly to the wheel-train structure of an electronic watch having a second hand.
A conventional electronic watch 100, which is disclosed in Japanese Patent Application No. 147940/1987, is illustrated in FIG. 3. Watch 100 includes a step motor having a coil 1, a magnetic core 2, a stator 4 and a rotor 5. A hairspring 10, which serves as the accumulator for rotational energy produced by the step motor, stores the rotational energy based on its elastic deformation. A control mechanism includes a viscous rotor 14 which is immersed in a viscous fluid 17 and which produces a load torque applied to a wheel train mechanism.
Coil 1 of the step motor generates a magnetic field for driving rotor 5 through magnetic core 2 and stator 4. Rotor 5 is coupled to a hair wheel 9 through a sixth pinion 6, a fifth gear 7 and a fifth pinion 8. Rotation of rotor 5 drives hair wheel 9. Hairspring 10 is connected to hair wheel 9 and a hairspring pinion 11. As hair wheel 9 intermittently rotates, an angular deviation between hair wheel 9 and hairspring pinion 11 is created due to the elastic deformation of hairspring 10. As hairspring 10 begins to recoil, producing a restoring (recoil) torque, hairspring pinion 11 begins to rotate. A second hand 16 is coupled to hairspring pinion 11 through a fourth idler 12 and fourth wheel 15. Rotation of hairspring pinion 11 causes second hand 16 to rotate. Viscous rotor 14 is coupled to fourth wheel 15 through a viscous rotor idler 48 and a rotor spindle 13.
As viscous rotor 14 rotates, a load torque proportional to the angular velocity of viscous rotor 14 is produced based on the viscous friction between viscous rotor 14 and viscous fluid 17. The load torque serves to regulate (i.e. control) any change in the rate of speed at which second hand 16 rotates. More particularly, as the recoil torque stored in hairspring 10 increases based on the difference in rotational frequency between hair wheel 9 and hairspring pinion 11, the rotational frequency of hairspring pinion 11 increases until it reaches a constant speed of rotation. Since the load torque of viscous rotor 14 changes in proportion to its angular velocity, the restoring torque retained by hairspring 10 as it increases results in increasing the angular velocity of viscous rotor 14. An increase in the viscous load of viscous rotor 14 results which opposes any increase in the angular velocity of viscous rotor 14. Since viscous rotor 14 is coupled through the gear train to hairspring pinion 11, any increase in the angular velocity of hairspring pinion 11 is also opposed. Similarly, when the torque retained in hairspring 10 decreases, any decrease in the angular velocity of hairspring pinion 11 is opposed by the viscous load of viscous rotor 14. Consequently, the speed at which second hand 16 rotates is maintained at a substantially constant level.
Fourth wheel 15 is coupled to second hand 16 through a second hand spindle 3. The wheel train mechanism is supported between a base plate 21 and a wheel train 22. Viscous rotor 14 is disposed within a cavity 19 which is integrally connected to base plate 21.
Another type of conventional electronic watch 200, which is disclosed in Japanese Patent Publication No. 47512/1981, is illustrated in FIG. 4. Watch 200 is driven by a quartz oscillator. Intermittent rotational energy is accumulated through magnetic attraction between a driving magnet 43 and a driven magnet 44. Driving magnet 43 is coupled to fourth wheel 15. Driven magnet 44 is immersed in viscous fluid 17. Rotation of driven magnet 44 in viscous fluid 17 transforms the intermittent rotational movement of driving magnet 43 into a relatively constant, uninterrupted rotational force. Driven magnet 44 magnetically interlocks with a follower magnet 45. A relatively constant, smooth rotation of second hand spindle 3, which is coupled to follower magnet 45, results.
Miniaturization of watches 100 and 200 are each limited by their construction. More particularly, since the accumulator and control mechanism of watch 100 are separated from each other a gear train mechanism including intermediate wheels 12 and 48 are required for linking the accumulator to the control mechanism. A relatively large area unsuitable for miniaturization results. Watch 200 requires the superposition of an hour hand spindle 47 on a minute hand spindle 40 for mounting the hour hand and minute hand, respectively. Fourth wheel 15 is coupled to a second wheel 39 through a third wheel 25. A second wheel train receiver 46 is also required for supporting and retaining driving magnet 43, driven 44 and follower (linkage) magnet 45. The construction of watch 200 makes it difficult to decrease its overall thickness and therefore is also unsuitable for miniaturization.
Accordingly, it is desirable to provide a timepiece having a second hand sweep-driven movement in which the wheel train structure is suitable for miniaturization and, in particular, to provide a timepiece having a second hand sweep-driven movement which is much thinner relative to conventional electronic watches.